Edward Zwick's Love and Other Drugs is a Frankenstein's monster, a movie bolted together from disparate parts that never properly co-ordinate with one another. It lumbers, breaks into an occasional trot, then falls apart completely. Zwick, a specialist in epic (Legends of the 31Fall, The Last Samurai), hasn't quite decided if he's making a socio-medical drama, a sex comedy, a drug-company infomercial or a tragic-illness movie – so we get an unholy mishmash of all four.

Jake Gyllenhaal stars as Jamie, a college drop-out who regards selling and seducing as interchangeable skills. He sweet-talks his way up the career ladder as a pharmaceuticals rep for Pfizer, just as he sweet-talks women into bed: he's clinical about it, like a junior version of George Clooney in Up in the Air. The time is the late 1990s, when Zoloft and Prozac are slugging it out as America's favourite happy pills, though the new drug on the block might have been designed for Jamie's brand of salesmanship: Viagra. "Who could sell a dick drug better than me?" I think this is the scriptwriters' notion of devilish charm. (They are adapting from the memoir of a former Viagra salesman, and nothing about the film would encourage you to read it.)

All is bowling merrily along when Jamie meets, in pretty unethical circumstances, a patient of his doctor friend. She is Maggie (Anne Hathaway), a 26-year-old waitress who's got early-onset Parkinson's Disease. There's a spark between them, and after some cajoling by him they get down to rampant sex, with Hathaway's boobs playing a part almost by themselves. Maggie is glad Jamie has confessed to being a "shit-head", because she'll never have to worry about his commitment or the prospect of being abandoned. Illness has made her that defensive. Not that you can tell she's ill; despite some minor hand-tremors, Hathaway never loses her lustrous skin-tone. When she's depressed, the film signals it by dressing her in a slightly less glamorous cardigan, the shade of oatmeal. Everything is calculatedly picturesque, right down to the massive bohemian-chic apartment she lives in – and can afford on her waitressing tips?

The sex and the salty talk may appeal or not – what's indisputable is its awkwardness in the context of a drama about disease. Jamie has a younger, fatter (actually obese) brother, Josh (Josh Gad), who's supposedly a hotshot financier but so hopeless in his personal life that he has to bunk down on Jamie's sofa. This leads to all kinds of smutty shenanigans, in which Josh doubles as the slob flatmate of sex-comedy tradition, either caught masturbating or making crude passes at unsuspecting women. But Zwick and co don't seem to understand how clueless this looks when the mood turns sombre at a convention of (actual) Parkinson's sufferers in Chicago, who talk about their affliction more honestly than the film knows what to do with. When Jamie is button-holed by a stranger who explains, with terrifying clarity, the awful future PD sufferers and their loved ones must prepare for, you feel a chill in your bones. It's a fine scene, hobbled by the narcissistic triviality of the rest. The idea of a pharma-market huckster finding his soul could only work if we believed he had a soul to find. This is just a phoney, latter-day Love Story cynically doped with a long, free advert for Pfizer.

Icy conditions, brutal temperatures, fuel shortages. If you think Britain had it bad this month, try watching The Way Back, an epic account of survival amid the very harshest circumstances imaginable. "Nature is your jailer, and she is without mercy." Thus are the inmates of a 1940s Siberian gulag warned of their situation, and that's before they've been forced to go down the mines, otherwise known as The Very Maw of Hell. Janusz (Jim Sturgess), falsely accused and imprisoned under the Stalinist terror, takes one look at the lice-ridden, snaggle-toothed huddle of humanity around him and sets his mind on escape. Along with him go a ragtag company, including Ed Harris as a hard-bitten Yank, Colin Farrell as a thuggish Russian patriot and Saoirse Ronan as a Polish waif.

Based on a gulag memoir by Slavomir Rawicz, whose escape took him on a 4,500-mile trek across Siberia, Mongolia – a wrong turn at China – the Himalayas and India, this is triumph-of-the-spirit filmmaking that feels a bit of a footslog in itself. The director, Peter Weir – no stranger to the wide-screen canvas (Witness, Master and Commander) – pays the amazing scenery its due, and conveys a little of how it might feel to be tested to the limits of endurance. No sooner have the escapees braved the murderous cold of Siberia than they are plunged into the skin-crisping heat of the Gobi desert.

As the journey drags on there is heartbreak, though it didn't prevent me glancing at my watch. "They haven't even reached the Himalayas yet," whispered my fidgeting companion. But we put our heads down and pressed on to the end. It's not that you doubt the sincerity of Weir's dedication to the story – but it is a story, rather than a plot. And with so little variation (apart from the weather) in its telling, The Way Back seems a great deal longer than its 133 minutes. By the time I had trudged home, it felt as though I hadn't seen my own front door in years.